There are numerous situations where it is important to control light energy and light transmission. For example, in a xerographic reproduction system, such as used in a printer, digital copier or facsimile machine, light from a source must be modulated into a series of dots (on and off conditions of the light) to form the image which is to be reproduced. The actual reproduction to the final media (typically paper) is accomplished through a light-sensitive photoreceptor in the form of a rotating drum, or belt, onto which the modulated light image has been transmitted. The drum is electrostatically dis-charged at the place where the light dots have exposed the drum so that charged ink particles, called toner, adhere to the drum at those places corresponding to the specific type of development process that is implemented. This toner is then transferred to the paper to create the final reproduced image.
In one system, it is desired to begin the process with an ordinary light source such as, for example, a tungsten halogen light bulb, and to reflect the unmodulated light rays from the light source off of a monolithic substrate which has built into it the ability to selectively reflect the light at particular locations on the chip. More specifically, the chip has on it at least one row of small deflectable mirrors which, when undeflected, reflect the light from the light source away from the drum. However, deflection of any, or all, of the individual mirrors causes the deflected mirror to image the light from that mirror location onto a corresponding position on the drum. At any one time, then, a group of deflections will cause an image of bright dots, called pixels, to be transmitted to the drum and ultimately printed onto the paper.
It goes without saying then that the ability of the system to accurately capture the proper reflected light and to reject all other extraneous light reflections is critical to the proper performance of the process. However, this is often difficult to accomplish, since stray light is particularly difficult to control. Compounding the problem is the fact that the light source must have enough energy to allow for all reflection and transmission losses. While theoretically, it is possible, problems still arise with the substrate mounting and alignment, and with the substrate itself which reflects light. Finally, the illuminating arm of the optics system must transmit a uniform irradiance profile to the modulator substrate to work properly.
One method of light control is to use baffles and light absorbing material. Thermal stress, however, can cause warping and other misalignment problems if too much heat, or IR energy, is absorbed by the various elements. Additional problems arise when it is desired to funnel the proper reflected, modulated, light dots to the drum and to mask all other light from the drum. This is because the ratio of modulated light to total light is relatively small (&lt;1%). Light absorbing and baffling problems are many faceted, and, if not carefully structured, will require complicated "tuning" and time consuming manufacturing techniques to achieve proper signal-to-noise ratios at the drum.
Since the modulated light must hit the drum in an exact pattern, and since all other light must be shielded from the drum, a system must be established which allows for the controlled disbursement of unwanted light rays and at the same time assuring precise handling of the proper modulated light signals. The system is further complicated by the fact that the modulated signals form a microscopically fine pattern that is changing extremely rapidly, on the order of 1720 times per second, and thus light paths must be closely controlled and repeatable. On the other hand, the stray light may continuously expose the photoreceptor surface over an area, or aperture, 25 times wider than the area exposed by the modulated pattern. Hence, a background wash of unwanted stray light at a very low intensity compared to the modulated signal can totally degrade the printed output. Because of the close tolerances, light divergences, which come about because of the turning mirrors in the transmission path, must also be controlled. All of these problems must be solved in an economical manner and without complicated alignment and trial-and-error light baffling techniques.
Thus, there is a further need in the art for such a system which allows for the control of the unwanted light energy without absorbing excessive heat, and without establishing complicated light transmission paths and precision component placement requirements.